This page explains how Confluence can change the look and feel by modifying CSS.
Cascading Style Sheets (CSS) are an industrial standard for styling Web pages. The content in the web page will be converted into HTML language, and the display and appearance of the page need to be defined through CSS files. You can upload a CSS text file, or you can type a stylesheet directly and apply the stylesheet to a space or the entire Confluence site.
Note: By default, only system administrators can edit CSS style sheets for the entire Confluence site. To allow anyone with space administrator privileges to edit a space's CSS stylesheets, go to 
> General Configuration > Security Configuration and select Custom Stylesheets for Spaces .
Creating a CSS file and being able to apply this CSS file to different browsers without bias is a basic work of website design and a delicate work. It's important to do specific testing of your CSS-modified Confluence site to make sure that all pages display properly - for example, modifying the CSS to make the Confluence main panel display properly, as well as all the base pages. normal display.
To help you with this modification, we created this help page: basic styling tutorial to give you some basic understanding of CSS.
https://www.cwiki.us/display/CONFLUENCEWIKI/Styling+Confluence+with+CSS